The metallurgy of tungsten plays a central role in the development of lamp filaments. Tungsten wire is made in various stages in accordance with the well-known Coolidge method, U.S. Pat. Nos. 1,082,933 (1913) and 1,226,470 (1917). Tungsten wire, which is used in the filaments of incandescent lamps, is subject to high mechanical loading and stresses, especially when it is used in lamps in which the filament operates at a temperature around 3000.degree. C.
Pure tungsten wire is not suitable to make filaments for incandescent lamps. Under typical operating conditions, the individual grains of the filament have the tendency to offset, or slide off (creep or sag) with respect to each other. This causes the filament to sag and short out. A lamp made with such filaments will, therefore, fail prematurely. The beneficial effects of doping to improve the creep resistance of tungsten wire were recognized as early as 1910, and doping was practiced henceforth. Systematic doping of tungsten oxide powder with potassium-containing chemicals was patented by Pacz in 1922, U.S. Pat. No. 1,410,499 (1922). Non-sag (NS) tungsten wire is unique in that it is a composite between two mutually insoluble metals, tungsten and potassium. The non-sag properties are attributed to longitudinal rows of sub-microscopic bubbles containing liquid and/or gaseous potassium.
The long chain of processes in a standard powder metallurgical (P/M) manufacturing of potassium-doped tungsten wire starts with the partial reduction of ammonium paratungstate tetrahydrate (APT), (NH.sub.4).sub.10 [H.sub.2 W.sub.12 O.sub.42 ].4H.sub.2 O, in hydrogen or hydrogen/nitrogen, which produces `tungsten blue oxide` (TBO), xNH.sub.3.yH.sub.2 O.WO.sub.n, where 0&lt;x&lt;0.1, 0&lt;y&lt;0.2, and 2.5&lt;n&lt;3.0. The specific composition of the blue-colored TBO depends on the reduction conditions: temperature, atmosphere, type of rotary kiln or pusher-type furnace and feed rate through the furnace. Along with crystalline compounds (WO.sub.3, W.sub.20 O.sub.58, W.sub.18 O.sub.49, WO.sub.2 and hexagonal tungsten bronze phases), the industrially produced TBO powders may contain up to 50% of amorphous phases. The TBO is doped with aqueous solutions of potassium silicate (1500-2500 ppm K, 1500-2500 ppm Si) and aluminum nitrate (or alternatively aluminum chloride) (.about.300 ppm Al). It is then dried and milled. The doped TBO is then reduced in hydrogen to metal powder. By some manufacturers a separate "browning" (reduction to .about."WO.sub.1 ") step is used. The doped tungsten powder is washed first with water, then with hydrofluoric and hydrochloric acid to remove unnecessary and undesired amounts of dopants. The powder is then dried in air. Appropriate powder blends are made to give a potassium content of .gtoreq.90 ppm in an acid-washed sample of powder. The washed powder is then mechanically or isostatically pressed and sintered by high-temperature resistance sintering at temperatures above 2900.degree. C. The ingots which have a density of &gt;17.0 g/cm.sup.3 and a K content of .gtoreq.60 ppm are rolled or swaged, and finally drawn into wire.
The multi-step process leads to the outstanding high-temperature creep resistance of NS tungsten wire. It is generally recognized that the NS tungsten wire should have a potassium content of at least about 60 ppm. Furthermore, it has been proposed that a potassium content of 80 ppm or higher, and in particular 85-110 ppm K, is necessary for high performance NS tungsten wire. K. Hara, et al., The Development of High Quality Tungsten Wire for High Stress Halogen Lamp, Nippon Tungsten Review 29 (1997), pp. 20-29.
With the conventional multi-step process retaining potassium is a challenge. Hence, it would be an advantage to have a method which could reliably achieve the incorporation of potassium in the ranges desired for high performance NS tungsten wire.